Various techniques for forming high performance metal alloys by powder metallurgy are known. Generally, these techniques involve forming a metal powder or particulate by atomizing a melt of the alloy, and then cooling the atomized alloy stream to effect solidification. Oftentimes, this process results in an alloy having a microstructure unachievable using more conventional techniques such as casting. It is thus possible to produce alloys having unusual physical properties.
Various applications might benefit from the improved physical properties afforded by metal powder materials. One of these applications is memory disk drives used in computers, commonly referred to as "hard disks" or simply "disk drives." Disk drives are formed of multiple parallel spaced disks each having a metallic substrate, usually nickel plated aluminum, with a magnetic coating. In use, the disks are rotated at high speeds, and an actuator arm scans over each disk to read and write digital programming.
Disk performance (i.e., memory capacity and read/write speed) is a function of the disk rotational speed. The faster the disks rotate, the greater the performance. Much effort has therefore been focused on the goal of increasing the disk speed.
However, achievement of this goal has been elusive. Currently, the upper limit on disk speed is approximately 7200 rpm using the industry standard 5000 series (5XXX) monolithic aluminum disk substrate. This upper limit is dictated by practical limitations on the amount of flutter permissible. Flutter is a phenomenon whereby the revolving disk begins to wobble above a certain rotational speed. If severe enough, flutter will result in the disk impacting against the actuator arm, which in turn may damage the disk and cause the disk drive to break down or "crash." Minimal flutter is therefore a prerequisite for preventing disk drive crashes and/or allowing higher disk speeds.
Several factors contribute to flutter. One is resonance caused by the interaction of the natural frequency of the disk and its rotational speed. The higher the natural frequency, the higher the rotational speed possible without resonance. The easiest way to increase the natural frequency of the disk, and hence "shift" the resonance point to a higher rotational speed, is to increase disk stiffness. Another factor influencing flutter is disk unevenness. Generally, the more uneven the disk surface, the greater the amount of wobble or flutter at a given rotational speed.
Hence, two desirable attributes of a metallic disk material are high stiffness and the ability to be highly machined and polished to a flat surface, thereby providing a high natural frequency and smooth surface finish. Furthermore, flutter can be attenuated by dampening factors contributed by composite materials. A ceramic material incorporated into a matrix (e.g. aluminum) provides a dampening factor which is a function of the interfacial surface area of the particles and the matrix. As the interfacial surface area increases, so does the dampening factor.
Various metal alloys and composite materials are known to have increased stiffness compared to monolithic aluminum. For several reasons, however, these materials are not wholly satisfactory for use in computer disks. For example, it is known to manufacture disks of silicon carbide or boron carbide in an aluminum matrix alloy. These composite materials are very stiff, having a modulus of about 14-30 msi. However, they are also very difficult to grind and polish to obtain a flat surface. Grinding rates are dramatically reduced compared to those for monolithic aluminum when using conventional grinding apparatus. This alone makes silicon carbide and boron carbide alloys impractical for commercial use in manufacturing computer disks. Another problem with polishing known prior art alloy materials is the hardness of the discontinuous phase. Grinding quickly dulls cutting tools and results in galling of the aluminum matrix. In addition, boron carbide or silicon carbide particles are literally pulled out of the aluminum matrix at the grinding surface, resulting in increased surface porosity.
In addition to minimizing flutter, a highly polished surface is necessary for proper coating of the substrate disk with a nickel plating. Without a highly polished surface, the plating will not be uniform. Gaps formed in the plating due to the lack of uniformity cause the subsequently applied magnetic coating to have imperfections, which in turn interfere with disk function.
A prior art process for forming metal alloyed memory disks is disclosed in U.S. Pat. No. 5,554,428. This patent teaches an aluminum alloy having various alloying elements such as zinc, copper, and dispersoid-forming elements of which scandium is one example. Disks are formed using casting and rolling.
U.S. Pat. No. 5,437,746 relates to a process for forming an aluminum alloy sheet which employs forming the aluminum alloy into an ingot or continuously cast thin sheet coil, followed by optional hot rolling, cold rolling and punching the alloy to produce a blank disk. The alloy includes magnesium, zinc, and copper.
The disks formed by these processes suffer from many of the disadvantages noted above. Hence, there remains a need in the art for a method for forming a disk having high stiffness and superior polishing characteristics. It would further be advantageous to manufacture actuator arms and various other components of a disk drive form the same or similar high stiffness alloy.